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Orbiting sensors study threat to ozone layer.

Despite glitches that delayed the delicate operation, crew members aboard the space shuttle Discovery last week deployed a satellite designed to vastly improve scientific understanding of Earth's protective ozone layer and the chemical pollution that degrades it. The successful release of the instrument-packed satellite kicked off NASA's "Mission to Planet Earth," a multi-decade effort to examine how human activities affect the global environment.

The largest U.S. satellite ever built for studying the Earth, the Upper Atmosphere Research Satellite (UARS) will probe the physical and chemical processes that dominate the stratosphere and mesosphere -- the two middle layers of atmosphere, extending from an altitude of about 15 kilometers up to 80 kilometers.

"We're trying to understand the processes so that we don't have any more surprises. The ozone hole, when it was discovered in 1985, was a surprise. People hadn't anticipated things like that," says UARS chief scientist Carl A. Reber of NASA's Goddard Space Flight Center in Greenbelt, Md.

The $710 million spacecraft, designed to operate for at least three years, carries 10 sensors. Four of the instruments measure atmospheric temperatures and concentrations of important chemicals. Two sensors record the speed and direction of stratospheric winds, which, among other things, set the stage each September for the development of the Antarctic ozone hole. Another four measure the energy deposited by the sun in the upper and middle atmosphere. This energy creates the ozone layer and powers the chemical reactions that allow chlorine pollution to destroy ozone.

The UARS sensors are not designed for long-term monitoring of ozone concentrations; rather, they will help scientists study the processes that control ozone levels. With instruments that can "see" between 80[degrees]N and 80[degrees]S, UARS will provide essentially global measurements of many factors previously studied only with aircraft and balloons, which collect information from limited regions of the atmosphere.

Perhaps most important, one instrument will provide the first nearly worldwide measurements of chlorine monoxide, the chemical that plays the principal role in creating the Antarctic ozone hole. Scientists have also detected extremely high levels of chlorine monoxide in the Arctic and elevated values over the United States, but they do not know the extent of these elevated levels.

The new satellite "is sort of a godsend," says Michael J. Prather, an atmospheric scientist at NASA's Goddard Institute for Space Studies in New York City. Prather leads an effort to determine whether fleets of high-speed, Concorde-like planes could damage the ozone layer. He and his collaborators hope to answer this and other questions about ozone loss by using computer models to simulate chemical reactions in the atmosphere. Measurements made by UARS will provde the data needed to test these models, he says.

NASA delayed the satellite's release from the shuttle when a check revealed a wiring mistake--detectable only in space--that caused its signals to have the wrong polarity. In two hours, engineers on the ground made simple changes to reverse the polarity, says project manager Charles E. Trevathan of the Goddard Space Flight Center.

But several hours later, UARS' primary transponder failed to tune in to a communications satellite that transmits information between the ground and UARS. Ground control switched to an identical, backup transponder on UARS, which successfully locked on to the communications signal. Soon afterward, the main transponder began working, says Trevathan. Engineers now seek to understand the problem.
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Title Annotation:Upper Atmosphere Research Satellite
Author:Monastersky, Richard
Publication:Science News
Date:Sep 21, 1991
Words:559
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